BRITANNIA LOSS PREVENTION DECEMBER 2023 THIS PRACTICAL GUIDANCE PROVIDES AN OVERVIEW OF THE KEY CONCEPTS AND CONSIDERATIONS INVOLVED IN CONDUCTING A DRAFT SURVEY FOR SHIPS. DRAFT SURVEY Draft surveys play a crucial role in determining the quantity of cargo loaded onto a ship. While based on the simple principle of Archimedes’ law of floatation, the calculation process can be complex due to the ship’s shape, non-linear changes in draft due to hog and sag, and external weather conditions. Simply put, the weight of the cargo is calculated between the difference of the weight of the ship before and after loading. Allowances are made for ballast and other known weights. Disputes can be prevented by managing the risk at the time of the fixture by clearly indicating which figure would be binding. USEFUL DEFINITIONS LIGHTSHIP This is usually the weight of the ship when it was built with operational conditions including the engine, accommodation and cargo space but does not include crew, cargo, fuel, ballast, fresh water and stores. This is thought to be increasing around 0.2% per annum due to the additional layers of paint, change of equipment and mud in the tanks. 2 | BRITANNIA LOSS PREVENTION | GUIDANCE CONSTANT Constant is the difference in weight of the calculated lightship (by draft survey without the cargo) and it’s tabular figures from the ships book at a given time. This is the result of the weight of crew, additional weight added to the lightship since new build i.e. rust and any added metal, mud, sludge and silt which is usually not accounted for. Therefore it is important to bear in mind that it is never constant and can vary. In practice the initial draft survey calculation can be used to work out the constant at that time. Often this calculated constant is compared with the declared constant or historical constant to sense check the cargo figures. This comparison may only be a warning flag of a possible error in the draft survey rather than an issue in itself. It is important to understand that the ‘constant’ may fluctuate, e.g. because of the variable amount of mud deposit. HOG Hogging occurs when the forward and the after end of a ship is much heavier than the midship. In mathematical terms, this means when the mean of forward and aft draft is bigger than the midship draft (ideally these should be same). 3 | BRITANNIA LOSS PREVENTION | GUIDANCE FACTORS TO CONSIDER DURING THE SURVEY READING THE DRAFT MARK SAG Sagging occurs when the midship section is heavier than the forward and aft. In mathematical terms, this means when the midship draft is bigger than the mean of forward and aft draft (ideally these should be same). All parties should read the draft close to the waterline to avoid parallax, for which a boat may be essential, and cost should be agreed beforehand. Mental notes of metre marks should be taken to avoid errors in metre reading. Always ensure that draft marks are equally spaced for accurate calculations. At the time of reading the draft, it is recommended to check for unusual flex conditions by checking all six point readings versus the means. While all ships hog/sag and sometimes twist, the actual flex tends to be typical in certain conditions (e.g. hogging in ballast condition) which is usually known by the chief officers. If after reading the draft, this is found to be against the norm, it could a warning sign of a possible error. In this case the reading of the draft should be double checked. Waves can distort th